In the past few decades our society's increasing demands for energy have naturally resulted in increased utilization of renewable reasources such as solar energy. One of the most common techniques of directly tapping solar energy involves the use of photovoltaic devices such as silicon solar cells. In general, solar cells are deployed in large solar arrays including numerous solar cells which are intricately positioned and interconnected to provide optimum electricity production.
A common problem in the use of solar cells is the requirement that they be protected from the terrestrial elements over a prolonged period of time. For example, in order to be commercially successful, solar arrays must be suitably encapsulated to achieve at least a 20-year lifetime expectancy in terrestrial environments. This has resulted in a concerted effort to develop a suitable encapsulating material to protect solar arrays from the elements over a sustained period of time.
Polymer films have been a natural choice as possible solar cell encapsulants. However, it has been a most difficult technical problem to develop proper polymers for encapsulating the solar cell arrays to protect the optically and electrically active elements from the degrading effects of typical terrestrial environments. In general, solar cell encapsulants have included three layers--the pottant, an adhesive and a weather resistant layer. The layer directly surrounding the solar cell is known as the pottant. The pottant insulates and protects the delicate mechanical and electrical elements of the solar cell against vibrations resulting from wind, earthquakes and other possible external forces. Ethylene vinyl acetate has found some success as a solar cell pottant, but it is many times difficult to apply.
The adhesive layer is necessary in order to secure the hard outer weather resistant layer to the relatively soft shock proof pottant layer. Many different adhesives, for example, silanes such as General Electric SS 4179 may be used for this purpose.
The weather resistant layer may be composed of different materials such as a hard acrylic polymer on the top and bottom surfaces and black or silicon rubber as a sealant along the edges. This layer functions to protect the solar cell from rain, dust and other debris.
In addition, technical problems in encapsulating solar cells are presented by the fact that it has long been desired to provide a method for directly encapsulating solar arrays at the final site. The encapsulation would be carried out after delivery and inspection of the numerous fragile electrical elements and interconnections and completion of testing to confirm that the electrical specifications have been met. Once it has been established that the solar array is in proper working order, the pottant medium is directly applied as a thin film over the solar cell array and further polymerized at the actual deployment site. The adhesive and hard outer coating are then applied. This severely limits the use of many polymers which are cast using environmentally undesirable solvents.
The likely candidates for solar cell pottants include silicones, fluorocarbons, and acrylic polymers. Acrylic polymers are preferred since they combine the essential factors of low cost, environmental resistance and general ease of processing.
Although acrylic polymers, in general, are preferred as solar cell pottants, the glass transition temperature for most acrylic polymers is too high, rendering them inappropriate for use in solar arrays subject to widely fluctuating temperatures. For example, below about 30.degree. F., methyl acrylate becomes hard and may crack. However, n-butyl acrylate has a sufficiently low glass transition temperature to render it unaffected by temperature fluctuations usually experienced by solar cell arrays. In addition, linear uncrosslinked n-butyl acrylate is very tacky and functions well as an adhesive. This would remove the need for a separate adhesive as part of the solar cell encapsulant.
Even though n-butyl acrylate has been determined to have the best overall balance of properties for use as a solar cell pottant, it has not been possible to readily process the n-butyl acrylate to provide economically feasible methods for encapsulating solar cell arrays.
In studies on polymerization of n-butyl acrylate, it was found that bulk polymerization is not suitable, because it is difficult or impossible to control the molecular weight and thus to prevent cross-linking of the polymerized product. Highly crosslinked n-butyl acrylate would be much too hard for use as a pottant. Solution polymerization was found to give a polymer having an unacceptably low molecular weight. Emulsion polymerization was also found to be unsatisfactory because the polymer product was found to contain various impurities such as soap and catalyst fragments which interfere with the operation of the solar arrays and cause premature weathering of the encapsulant.
One promising method for applying n-butyl acrylate to solar arrays involves the preparation of an n-butyl acrylate polymer syrup solution containing a mixture of n-butyl acrylate monomer and poly(n-butyl) acrylate prepolymer. A polymer syrup solution of this type would be specifically useful for rapid formation of transparent films or sheets and as a pottant for solar cells and other delicate components without the use of high cost capital equipment, including such facilities as those currently required to provide for solvent recovery and minimization of atmospheric pollution.
The use of polymer syrups is well known in the preparation of methyl methacrylate where sheet casting results in a hard thin film. Polymer syrups of acrylates have not been used in sheet casting because of the highly unmanagable tacky film which would result.
In general, uncrosslinked acrylic polymers are soluble in their respective liquid monomers. Therefore, a syrup solution of monomer and polymer is prepared which when poured or otherwise applied to a surface results in polymerization of the remaining monomer to form the desired polymer film. The presence of the polymer helps to absorb heat generated during the polymerization of the monomer, thereby preventing undesirable crosslinking. This type of syrup solution would be ideal for field applications as a pottant to solar arrays and general application to any substrate requiring protection.
Although it is highly desirable to product such an n-butyl acrylate polymer syrup, attempts to produce such a syrup have not been successful. For example, attempts were made using methanol as a solvent in preparation of the poly(n-butyl)acrylate prepolymer; however, crosslinking of the prepolymer occurred resulting in low solubility in the n-butyl acrylate monomer. Other attempts using a methanol/water solvent for preparation of the poly(n-butyl) acrylate prepolymer prevented undesirable crosslinking of the prepolymer, but resulted in undesirably low overall yields of linear polymers.